Next Contents Previous


There are two broad, interrelated questions concerning star formation in normal, non-interacting irregular galaxies whose answers would take us a long ways towards understanding global star formation processes and galaxy evolution. The first is ``What governs the overall rate at which a galaxy forms stars?'' Irregulars having very similar global properties nevertheless show a wide range in total star formation rates relative to their sizes. Secondly, ``What initiates star/cloud formation within irregular galaxies?'' Since spiral density waves, which were once believed to be the primary triggers for star formation, are not applicable in irregular galaxies, we must determine what processes do control star formation in these systems. These processes may also operate in spiral galaxies but be difficult to disentangle from the effects of the more dominant density wave. Thus, we need to learn what regulates star formation locally in irregular galaxies, what feedback mechanisms are operating and their relative importance, what consequences there are to the star formation process due to differences in the ISMs of irregulars compared to spirals, and what role the extended gas plays in the evolution of the optical irregular galaxy.

We find that star formation is a local or regional process in the sense that it is radially similar today to what it has been historically over large time-scales. Of course, star formation in small galaxies is a ``grainy'' process and there are stochastic variations on short time-scales, but integrated over billions of years star formation is regionally constant. In the context of a simple thin rotating disk model, the gas in irregulars appears to be more stable against cloud formation than in spirals, but none of the models in the literature correctly predicts the radial star formation pattern in irregulars. This suggests that the models are too simple and that other factors, such as feedback from massive stars, random gas motions, and lack of shear may play a larger role in facilitating star formation in irregulars than they may play in spirals. The signatures of feedback from concentrations of massive stars are frequently seen in the ISMs of irregulars and in some cases clear examples of star-induced star formation are seen as well. However, the collective global contribution of this feedback process to star formation in irregulars is not clear, although arguments have been made that the net global effect must be to inhibit further star formation.

In spite of differences between irregulars and spirals in terms of their ISMs, the stellar products of the star formation process and the efficiencies of turning a cloud into stars appear to be the same. Thus, the local process of star formation, what happens inside a cloud once the cloud has formed, is not a function of the type of galaxy. One consequence to differences in amounts of interstellar shear, however, is that irregulars can form larger gas clouds which collapse quickly to form giant H II regions. These concentrations in time and space of large numbers of massive stars will have a larger impact on the ISM than the same number of stars formed over a larger area and time period.

Most irregular galaxies have H I gas that is extended well-beyond the stars, and some have gas that is unusually extended - up to seven times the Holmberg radius. This far-flung gas is potentially a vast reservoir from which to fuel star formation for a very long time, but there is no evidence at present that these H I envelopes are having a major impact on the optical galaxy. However, we cannot rule out a very slow replenishment of the gas in the center of the galaxy as star formation depletes the gas there. A few of the unusually extended gas envelopes are now known to be highly structured, inhomogeneous, and not quiescent, and it is probable that those systems will change radically with time.

The issues of star formation and the ISM of irregular galaxies are related to other areas of astronomical research. Irregular galaxies are particularly underevolved compared to spiral and elliptical galaxies and so they pose an interesting glimpse of the early stages of galaxy evolution. Furthermore, their large, extended gas envelopes offer a chance to examine the role that such components of the galaxy may play in the development of the optical galaxy, and the extended gas may be connected to QSO absorption line systems. In addition dwarf spheroidals, such as those that are companions to the Milky Way, are believed to be fossils of the smallest irregular galaxies. Finally, the class of irregular galaxies referred to as Blue Compact Dwarfs, not discussed here, are considered to be candidates for the faint blue galaxy population seen at large look-back times and can possibly shed some insight into the galactic starburst phenomenum believed to account for the faint blue galaxies. Thus, irregular galaxies are a rich mine for furthering our understanding of galactic processes.


I would like to thank the Instituto de Astrofísica de Canarias for hospitality while this paper was being worked on. I am very grateful to R. Kennicutt, R. Larson, and P. Massey for comments on a draft of this paper.

Next Contents Previous